It is increasingly important in view of stricter environmental regulations concerning liquid and gaseous waste from industrial and municipal facilities to provide apparatus and systems that effectively and efficiently remove contaminants from such effluents and simultaneously degrade the removed contaminants. Many prior efforts in the field have concentrated on the often difficult task of separating contaminants from the liquid or gaseous carrier. However, mere separation of the contaminant from the carrier liquid or gas without disposal oftentimes does not satisfy regulatory requirements.
Also, separation creates the problem of disposal of the separated contaminant, which is increasingly a problem in view of increasing limitations placed on contaminant disposal. Therefore, it has been highly desirable to provide an apparatus capable of not only separating contaminants from effluent carriers, but also degrading the separated contaminants substantially simultaneously. Otherwise, alternate methods of contaminant disposal must be contemplated subsequent to separation. Unfortunately, many degradation apparatus and methods have proven unsuitable for combination with many current separation techniques whereby both may be simultaneously combined in an effective and efficient manner.
A commonly used vapor-phase simultaneous separation and degradation process is biofiltration. Biofiltration utilizes microorganisms immobilized in the form of a biofilm layer on a porous, moist, organic filter packing such as compost, peat or wood chips to oxidize vapor-phase contaminants completely to carbon dioxide, water and mineral salts at ambient conditions. Biofilters have been used in Europe for over 30 years to control odorous air emissions. Biofilters have also been used in the United States to treat hydrogen sulfide, alcohols and other odor-causing air-borne contaminants emitted from wastewater treatment plants, industrial process streams and composting facilities. Recent advances in biofilter technology have expanded the range of treatable target compounds to include many odor-causing VOCs and air toxics as well.
Another type of biological system used to treat air-borne contaminants is a biotrickling filter. Biotrickling filters are similar to biofilters but contain scrubber packing material instead of compost, peat or wood chips and operate with liquid flow over the packing co-current or counter-current to the flow of vapor. Biofilters are typically not operated with continuous liquid flow over the packing.
The pH of the recirculating liquid within biotrickling filters is monitored and controlled by the automatic addition of acid or base. However, the pH within biofilters is controlled by the addition of solid calcium carbonate or lime to the packing material at the beginning of operation. Once this buffering capacity is exhausted, the filter bed is removed and replaced with fresh material. Biofilter bed replacement can be quite frequent for the biodegradation of halogenated contaminants. Therefore, biotrickling filters are often more cost effective than biofilters for treatment of halogenated contaminants such as methylene chloride.
However, biotrickling filters suffer from operational drawbacks in that the packing material is subject to buildups of biologically active materials, crusting, clogging and the like. This clogging usually occurs where the vapors enter the biotrickling filter, where most of the biological growth occurs. The result is uneven vapor distribution, vapor and/or liquid channeling and/or reduced vapor residence time-all of which are highly detrimental to system performance. Elimination of these drawbacks via prevention and removal of the buildups is a main object of this invention.